I

Q;o: Physical Properties and Superconductivity

William R. Entley Literature Seminar September 26, 1991

In 1985, Smalley discovered that the laser vaporization of graphite in a pulsed jet of he­lium spontaneously forms Ctio in a condensing carbon vapor [1]. Under cenain experimental conditions Qio is essentially the only carbon species formed and Smalley proposed that C6Q must adopt a special structure in order to account for this unusual selectivity. The structure he proposed was a truncated icosahedron, known informally as "Buckminsterfullerene" (Fig. 1). ln 1990, Kriitschmer reponed a simple macroscopic preparation of this first reponed crys­talline molecular allotrope of carbon [2], and subsequently a flurry of activity in research labo­ratories around the world was initiated. The originally proposed truncated icosahedral struc­ture of C6o was strongly supponed by 13c NMR [3], IR [2], and Raman spectra [4], how­ever determination of the exact structure of the molecule by single crystal X-ray diffraction was frustrated by the rapid isotropic rotational motion of the spherical molecules in the crystal down to temperatures of 100 K [5]. 111is obstacle was overcome by osmylating C60 thus pro­ducing rotationally ordered crystals of a 1: I Q;o osmium tetra.oxide adduct. The resulting sin­gle crystal X-ray structure of this Q;o derivative confirmed its soccerball geometry [ 6].

Figure 1: Truncated icosahedral structure of Buckminsterfullerene

Hiickel molecular orbital calculations carried out by Hadden in 1986 suggested that C60 would exhibit an exceptionally high electron affinity, reflecting its relatively low-lying triply degenerate 7t tlu LUMO [7]. This prediction was confinned experimentally: cyclic voltam­metric studies show that Q;o readily undergoes three reversible one-electron reductions [8]. Other researchers have shown that Q)o readily undergoes a Birch reduction to C6QH36 [9], and that it produces deep red-brown solutions of diamagnetic polyanions upon reaction with lithium in tetrahydrofuran [10]. The electron deficient character of Q)o determines much of its chemistry, and is reflected in its reactivity towards low-valent late transition metals, such as Pt(O) [10] and Ir(I) [11].

In situ measurements of the electrical conductivity of Q)o films doped with alkali metal vapors establish that the doped films behave as organic metals and exhibit conductivities at ambient temperature comparable to those of n-type doped poly acetylene. Doping to saturation, however, always results in the formation of an insulating phase [13]. Low temperature inves­tigations of the highly conducting potassium-doped phase, in the form of both thin films and polycrystalline powders, indicate that the highly conducting state undergoes a phase transition to a superconducting state at 18 K (Fig. 2) [14]. Carefully controlled reactions establish there is a single superconducting phase of composition K3Qio [15]. Structural data are consistent with this formulation, corresponding to complete filling of all the interstitial sites in a face cen­tered cubic lattice of Q;o units (16]. Photoemission [17], 13c NMR (18], and Raman spec-

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troscopy [13] as well as powder X-ray diffraction data [19] support the formulation of the in­sulating phase as Kt>Oio. which adopts an intercalated body centered cubic structure [19].

0.0

--;- - o j Ol

::i

E-o 2 - • • ••••• Qi . .., ' =-0.3 :t ~ -o t;

. : ..... -.

~ ··

FC

/ ZFC

-

- 0.5 ...._ __ __.~_.___._~...._--__.~~-----

o 5 10 ~ 5 20 25 T !Kl

Figure 2: Temperature dependent magnetization of a KxCoo crystalline sample. The direction of temperature sweep in the field-cooled (FC) and the zero-field-cooled (ZFC) curves is

indicated by the arrows.

A key investigation of the pressure dependence of the superconducting transition tem­perature in K3C6o established that dTcfd.P = -0.78 K/kbar. This decrease in the critical tem­perature CTc) with increasing pressure is unusually large for a superconductor and is entirely reversible upon decreasing the pressure [20]. The critical temperature is also dependent on the size of the alkali metal dopant and increases significantly if larger alkali metals are used (21]. These trends in the variation of Tc can be rationalized in terms of the Bardeen-Cooper­Schrieffer (BCS) theory of superconductivity, which in its simplest form leads to the following quantitative relation,

Tc= 0dexp[-1NN(Ep)]

where 0d is the Debye temperature, Vis the electton-phonon interaction, and N(Ep) is the density of states at the Fermi level [22]. Since increasing the pressure decreases the density of states at the Fermi level, while doping with successively larger alkali metals increases the den­sity of states at the Fermi level, BCS theory accounts qualitatively for the changes observed in Tc· Extended Hiickel band structure calculations of the density of states for the alkali doped fullerides account almost quantitatively for the observed changes in Tc on doping with succes­sively larger alkali metals and suppon the conclusion that the density of states at the Fermi level is the controlling factor that governs Tc [23].

The reduced state adopted by the superconducting phase is consistent with the electron deficent character of Coo which defines much of its chemistry. Although the mechanism of su­perconductivity in Q;o is not established with certainty, the evidence suppons a conventional BCS type mechanism. Further advances leave open the possibility of even higher supercon­ducting transition temperatures.

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References

1. Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F .; Smalley, R. E., "C60: Buckminsterfullerene," Nature 1985, 318, 162-163.

2. Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R., "Solid c60: A New Form of Carbon," Nature 1990, 347, 354-357.

3. Johnson, R. D.; Meijer, G.; Bethune, D.S., "C6o has Icosahedral Symmetry," J. Am. Chem. Soc. 1990, 112, 8983-8984.

4. Bethune. D. S.; Meijer, G.; Tang, W. C.; Rosen, H.J.; Golden, W. G.; Seki, H.; Brown, C. A.; Vries, M. S., "Vibrational Raman and Infrared Spectra of Chromato­graphically Separated C6o and C7o Fullerene Clusters," Chem. Phys. Lett. 1991, 179, 181-186.

5. (a)

(b)

Yannoni, C. S.; Johnson, R. D.; Meijer, G.; Bethune, D. S.; Salem, J. R., "13C NMR Study of the C60 Cluster in the Solid State: Molecular Motion and Carbon Chemical Shift Anisotropy," J. Phys. Chem. 1991, 95 , 9-10. Tycko, R.; Haddon, R. C.; Dabbagh, G.; Glarum, S. H .; Douglass, D . C.; Mujsce, A. M., "Solid-State Magnetic Resonance Spectroscopy ofFullerenes," J. Phys. Chem. 1991, 95, 518-520.

6. Hawkins, J.M.; Meyer, A.; Lewis, T. A.; Loren, S.; Hollander, F . J., "Crystal Structure of Osmylated C6Q: Confirmation of the Soccer Ball Framework," Science 1991, 252, 312-313.

7. Haddon, R. C.; Brus, L. E.; Raghavachari, K., "Electronic Structure and Bonding in Icosahedral C6o,'' Chem. Phys. Lett. 1986, 125, 459-464.

8. Allemand, P. M.; Koch, A.; Wudl, F., "Two Different Fullerenes Have The Same Cyclic Voltarnmetry,'' J. Am. Chem. Soc. 1991, 113, 1051-1052.

9. Haufler, R. E. ; Conceicao, J. ; Chibante, L. P . F.; Chai, Y.; Byrne, N. E.; Flanagan, S.; Haley, M. M.; O 'Brien, S. C.; Pan, C; Xiao, Z.; Billups, W. E .; Ciufolini, M.A.; Hauge, R. H .; M argrave, J. L.; Wilson, L. J.; Curl, R. F .; Smalley, R. E., "Efficient Production of C60 (Buckminsterfullerene), C6oli36. and the Solvated Buckide Ion," J. Phys. Chem. 1990, 94, 8634-8636.

10. Bausch, J. W.; Surya Prakash, G. K; Olah, G. A., "Diamagnetic Polyanions of the Coo and C7o Fullerenes: Preparation, 13c and 7Li NMR Spectroscopic Observation, and Alkylation with Methyl Iodide to Polymethylated Fullerenes," J. Am. Chem. Soc. 1991, 113, 3205-3206.

11. Fagan, P. J.; Calabrese, J. C.; Malone, B., "The Chemical Nature of Buckminsterfullerene (C(i()) and the Characterization of a Platinum Derivative," Science 1991, 252 , 1160-1161.

12. Koefod, R. S.; Hudgens, M. F.; Shapley, J. R., "Organometallic Chemistry with Buckminsterfullerene. Preparation and Some Properties of an Iridium(!) Complex.," submitted for publication in J. Am. Chem. Soc.

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13. Haddon, R. C.; Hebard, A. F.; Rosseinsky, M. J.; Murphy, D. W.; Duclos, S. I.; Lyons, K. B.; Miller, B.; Rosarnilia, J. M.; Fleming, R. M.; Kortan, A. R.; Glarum, S. H.; Makhija, A. V.; Muller, A. J.; Eick, R. H.; Zahurak, S. M.; Tycko, R.; Dabbagh, G.; Theil, F. A., "Conducting Films of C60 and C70 by Alkali-Metal Doping," Nature 1991, 350, 320-322.

14. Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D . W.; Glarum, S. H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R., "Superconductivity at 18 Kin Potassium-Doped C60," Nature 1991, 350, 600-601.

15. Holczer, K.; Klein, 0.; Huang, S.-M.; Kaner, R. B.; Fu, K-J.; Whetten, R. L.; Diederich, F., "Alkali-Fulleride Superconductors: Synthesis, Composition, and Diamagnetic Shielding," Science 1991, 252, 1154-1157.

16. Stephens, P. W.; Mihaly, L; Lee, P. L.; Whetten, R. L.; Huang, S.-M.; Kaner, R.; Diederich, F.; Holczer, K., "Structure of Single-Phase Superconducting K3C60," Science 1991,252, 1154-1157.

17. Benning, P. J.; Martins, J. L.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E., "Electronic States of KxC60: Insulating, Metallic, and Superconducting Character," Science 1991, 252, 1417-1419.

18. Tycko, R.; Dabbagh, G.; Rosseinsky, M. J.; Murphy, D. W.; Fleming, R. M.; Ramirez, A. P.; Tully, J.C., "13C NMR Spectroscopy of KxC60: Phase Separation, Molecular Dynamics, and Metallic Properties,'' Science 1991, 253, 884-886.

19. Zhou, O.; Fischer, J.E.; Coustel, N.; Kycia, S.; Zhu, Q.; McGhie, A. R.; Rommanow, W. J.; McCauley, J.P. Jr.; Smith, A. B. III; Cox, D. E., "Structure and Bonding in Alkali-Metal-Doped C60," Nature 1991, 351, 462-464.

20. Spam, G.; Thompson, J. D.; Huang, S.-M.; Kaner, R. B.; Diederich, F.; Whetten, R. L.; Griiner, G.; Holczer, K., "Pressure Dependence of Superconductivity in Single­Phase K3C6o," Science 1991, 252, 1829-1831.

21. Rosseinsky, M. J.; Ramirez, A. P.; Glarum, S. H.; Murphy, D. W.; Haddon, R. C.; Hebard, A. F.; Palstra, T. T. M.; Konan, A. R.; Zahurak, S. M.; Makhija, A. V., "Superconductivity at 28 Kin RbxC60," Phys. Rev. Lett. 1991, 66, 2830-2832.

22. Myers, H.P. Introductory Solid State Physics; Taylor and Francis: Philadelphia, 1990; Chapter 13.

23. Fleming, R. M.; Ramirez, A. P.; Rosseinsky, M. J.; Murphy, D. W.; Haddon, R. C.; Zahurak, S. M.; Makhija, A. V., "Relation of Structure and Superconducting Transition Temperatures in A3C60," Nature 1991, 352, 787-788.

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